The scanning acoustic microscope (SAM) has become a convenient and familiar tool for inspection and quality control in manufacturing and other industrial applications. One of the more effective devices of this kind employs a C-mode scanning mechanism, with the object under investigation insonified by a series of ultrasonic acoustic pulses; ultrasound reflections from the object are received and are utilized to generate an initial electrical signal. That electrical signal, in turn, is used to develop an image of the object, an image that can focus on varying transition levels within the object. A basic system of this kind is described in the article "Using the Precision C-SAM for Bilateral Inspection of Die Attach" in Microelectronic Manufacturing and Testing, June, 1987.
One diffuclty in operation of a reflection-mode SAM has been distinguishing "hard" discontinuities from "soft" imperfections. A "hard" discontinuity is a solid that is more rigid than adjacent material in the object, whereas a "soft" imperfection may be a spongy discontinuity or even a gas pocket or void. In most SAMs it is difficult to distinguish between the two because the magnitudes of the ultrasonic echoes from them may be similar or even essentially equal, though the phases of the echoes are distinguishable. Some SAMs have included means to switch from an image of one phase (polarity) to an image of the other phase. This is not really satisfactory, however, because the investigator using the SAM cannot see a unified image of all of the available information.